Radio transmission systems



G. D. GILLETT RADIO TRANSMISSION SYSTEMS Filed June l0. 1930 Oct. ll, 1932.

ATTORNEY vBy Patented Get. l1, i932 entre stares GLENN E. GILLETT, or ENGLEWOOD, NEW JERSEY, AssIGNoR To BELL TELErnoivn LAEoRAToRrEs, INCORPORATED, or NEW YORK, N. Y., A coRroRarroNfoE NEW YORK RADIO TRANSMISSION S-YSTEMS Application filed June 10,

This invention relates to radio transmission systems and particularly to systems such as chains of broadcasting transmitters 1n Y which two or more transmitters operate at a common frequency.

One object of the invention is to improve the synchronization of radio transmit-ters radiating waves at a common frequency.

Another object of the invention is to increase the area that'can be effectively served by a plurality of broadcasting transmittersy operating at a common frequency and broadcasting the same program.

Ilhe problem of maintaining broadcast coverage of a given area by means of a plurality of reiatively low power transmitters operating at a common frequency is complicated by the occurrence in the area of vzones in which the waves from the several stations i interfere and produce a neutralization of is impracticable. At other vpoints where the e field intensity of one wave is predominately great, the degree of fading may not-be large but, if rapid enough, will so distort the signals that clear reception is impossible. The result of this is that thel area that can be served by each station is greatly restricted when both stations are operated together.

It has been found that if the fading period is not permitted to fall below a certain optimum value, the distorting effect is noticeable only in the very small area where the waves have about equal amplitudes. Consequently by maintaining the transmitters in `such isochronism that the fading period is of this optimum duration, the area that can be served by each station, when the two are operated together, is practically asgreaty as the area that is served by each when op- 1930. Serial No. 460,246.

erating separately. Experience hasshownj` c period is of the quencycontrolling means are used at each station to furnish the carrier waves. 'lihese oscillators are stabilized by known methods to secure a high degree of constancy in the frequency of the generated waves. By the use of the quartz crystal oscillators'along the isochronism may be maintainedwithiny afew parts in a million, ybut this is insufiicient for the prevention of interference. To ef-v fect'the more exact isochronism, a radio receiver is located between the two stationsV at a point where the field intensities are about equal. This receiver is tied to one transmitting station by a wire line over which the detected signals are transmitted to a loud speaking telephone receiver or other suitable indicator. If t-he stations are not in isochronism, the fading period of the detected signals provides an indication of the frequency dierencc and by proper adjustment of vth-e oscillator'control, the fading period canbe -maintained at an unobjectionable value.

In order to make the received beat independent'of the character of the broadcast signal and to prevent a discrimination to be I made between interference fading and musical rythm, an auxiliary low frequency tone,

may be introduced into the controlv receiver in such a manner that its amplitude in the receiver output follows the variations in the amplitude of the combined carrier waves.

Theinvention may be morereadily understood by reference to the following detailed description in connection with the drawing,

in which Y Fig. l shows diagrammatically the circuit arrangements of one embodiment of the invention;

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' Fig. 2 shows Vdiagrammatically 'the geo,- graphical arrangement of the transmitters and control receiver of Fig. 1; and

Fig. 3 shows diagrammatically the geo# graphical arrangement of the transmitters and control recei-ver of amodification of the invention employing a directive antenna at the control receiver.

Referring tovlig 1,V there areshowntwo similar"V radiol broadcasting 'transmitters' '10' andf20.A Thet,ranemitter 10 per se comprises an oscillator 11, the frequency of .which is controlled by a quartz crystal "1-2,"an audiofrequency amplifier 13, a modulating ampli; ier 14 and a power "amplifier 15,"tle Voutputof which is connected to `an antenna 16. There is connected across the quartz crystal 12 a variable condenser 17 which permitsaf sensitive adjustment ofthe frequency of the generatedl wave, A ,volume indicator 18is connectedto the input of the modulatingamplifier to permit a; measurement ofthe amplitude of the audio frequency waves impressedv thereon. A potentiometer 19 connected to `the output of the audio' frequency amplifier 13. permits-the control of the amplitude of the audio frequency wave impressed en the modulating amplifier. As is the usual prac,

tice, a sensitive temperature .control (not (shown) isprovided' for the crystal 12.

United. stm-,es Patentime? to Man-i- Y son, May 6, 1930, 'discloses one type of temperature control which h I l as been found vsatisfactory for such purpose. Y' V'By means of the switches 31 and 32, the input of the audio frequency ampliiier 13 be connected either to a studio equipment 40l or to-atelephone line 33. The studio equip- Y mentY 40 is of the conventional type ,compris-f- Ving a pick-up microphone 41an audio fre# quency amplifier 42, a volume control 43, and

, a .volume indicator 44.' d

that ,provided for transmitterA 10, the input` of thel transmitter may beconnected by. means of the switches 61 vand 62 either to av The transmitter v2O is identical to the transmitter 10 and consequently will not be de-V scribed in detail. 'In a similar manner to Studioequipment 5 0 orto .the telephone line This line is provided withtelephone're-v peaters orampliiers 53 andc54." The studio equipment 50 isv similar to the studio equipment 40. Y l

I/fThere is alsoprovidedaY radiofreceiver 70 :which is locatedA at a point .intermediate the. transmitters 10r and 20a`s willbe later de' vscri-hed in detailwith reference to Fig. 2. Thlsreceiver comprises ank antenna 71, 'coupled' to. a modulator 72, the output of which is connected to an audiofrequency amplifierA 7 3,.wh1ch lnkturn operates into aV detector 74.'

An audio frequency generator 7 5, which generates a suitable tone, for example, of av freq'ueney of 150 cycles per second is also coupled to the modulator 72. The output of `thev frequencies of onev` or both carriers.

1,881,483 jf l' j the detector 74 is connected to a telephone or telegraph line 95 which interconnects the Vpoints at which the receiver 7 0 and the transmitter 10 are located. At' the transmitterV l 10 this line 95 is connected toar loud speaker 7,6v and to. the'finput of a,v rectier 77, the out-A put of which is connected toan indicating meter such as a milliammeter 78.' As 'will be later described. inconnection with the operaf tion of the" system, either ofthe loud speak- 'ing vtelephone receiver" 7 6 'orthe :rectifiern indicator combination 77, 7 8 or both, may be empl'oyedft* Y The two .radiotransmitters 10v and 20'are program from eitherfof the studios or VV may be transmittedwfrom bothl transmitters-by the proper adjustment of the-switches 'Y 31, 32,.`61, and 62.5V ForeXamplai-f itis desired to transmit a program from the studio Y' 40, switchesnl', 32 and 62 will be closed, and the switchlv 61 Opened., -,Under these condi` tions the transmitters 10 and 20 will beltransmitting the same program at the same carrier frequency. 1 ,l j Y A As willfloelaterdescribedr in detail in vconnection with Fig.' 2, the receiver 70 vislocated atfrsuch a pointthat the amplitudes of the car1'ier,`waves from the transmitters 10 and f 20rece1ved" in the antenna 71, are substantially equal. These received carrier waves' Vare. modulated. in the'modulator 72 by the audiofrequency wave from the generator 75.

The resultant Wave is Vamplified inthe ampli-fier 73 and. detected in the-detector `74.

The detected Wave is transmitted back tothe'V station 1OfloyA the-line 95. Atthe station 10,` the detectedY wave gives an audible indication in the loud 'speakingA receiver 76 and a visualY indication in therindicating meter 78.

As the frequency ofthe carrier Wave of one or both the transmitters 10- or 20 varies to throw vthe.twolwtran'smitters out of isochronism, the' output'A of the detector.' 74' will go through ay fading cycle, the `frequency of which isthe actual diil'erence in the frequencies oflthe'carrier Awaves of the two transmitters. VBly'observing the frequency of this f fading cycle as audibly indicated in the receiver 76, for eXample,the operator at sta-- tion 10 is able' teso adjust,'by meansof the variable condenser 17, thefrequency of that. *"5

carrier wave thatthe fading period is unobjectionable."v f l VIt has been found practice that when the transmitters V`have veryaccurately calibrated-crystal. controlled oscillators, it is possibly byA a manual adjustment as just described to hold the transmitters so closely in isochronism that the period of fading of the signal output of the detector 74 is of the order of several minutes. Vhile such a close isochronism would appear to be the ideal condition, it has been found thatwith such a control the receiving conditions in the interference zone, which can never be entirely eliminated, are very objectionable. The reason for this is that such a long fading period will tend to wipe out entire phrases or sentences in the received program, thus making it unintelligible. By controlling theV isochronism so that the fading period is of the order of ten to thirty seconds, the interfering zone can be kept almost as small as with the more accurate isochronism and the receiving conditions in that zone will be such that only an occasional word will be missed. lt has been'found that such a receiving condition is n-ot at all unobjectionable.

If it is desired the modulator 72 and the tone generator 7 5 may be omitted from the receiver 70. lVith such a circuit the output of the detector 74 contains the fading signal as in any ordinary broadcast receiver located in the interfering zone. When so operating the system, the control operator at the station 10 can control the frequency of the oscillator 11 by observing the fading period of the signal itself. However, the fading of the signal ma-y be of the same order as the musical rhythm of a program being transmitted, and consequently the proper control of the oscillator may be rather diiiicult. With the preferred circuit embodiment shown, the amplitude of the audible tone in the generator 75 can be made such that the tone can be easily discriminated from any signal which is being transmitted.v In addition by the use of the signal tone of the order of 150 cycles it is possible to use a line for the transmission line 95 which has only a narrow frequency range such as a telegraph channel. Since the cost of such lines is considerably less, the use of the audible tone gives an economical advantage.

Fig. 2 shows the geographical arrangement of the units of the system of this invention. The two transmitters are indicated by the rectangles 10 and 2O respectively. The areas 8l and 82 represent the areas in which good quality reception can be effected with either transmitter operating alone.

When the two transmitters are operating at the same frequency with only the ordinary individual controls, the areas in which good reception is possible are indicated by the inclosures 83 and 84. Under such conditions reception at all points outside of these areas is practically impossible. y

When employing the control system of this invention, good quality reception is possible at all points within the two inclosures 81 and 82 except in the cross-hatched portion where noticeable interference effects will be observed. However, even within this area, it is possible to obtain fair reception and by the use of receivers having directive antennae such as loops so that one' transmitter may be discriminated against, it is possible to obtain quite good reception.

As indicated in the diagram by the rectangle ?0, thereceive'r 70 is located at a point at which the amplitudes of the carrier waves from the two stations are substantially equal. n the ideal condition inwhich the terrain is uniform andthe two transmitters employ similar antennae, giving uniform fields, this point would be at equal distance' from the two stations. f

In practice it lmay be very difcult to locate a point at which the amplitudes'of the carrier waves are exactly alike or when such a.l point can be located, it may be a very inconvenient point at which to locate the control receiver. This difficulty can be overcome by the use of a directive antenna-,for example,

a loop orV double loop. When employingy such an antenna it is possible to locate the control receiver at a point such as 7 0 and 'toL so adjust the antenna that thel amplitudes of the received wavesimpressed upon the receiver are equal. Fig?) shows diagrammatically such an arrangement. In this figure the rectangles 10 andl 2O the two transmitters, asin Fig. `2, and the .point 70 represents the vlocation of the control receiver, as also shown in F ig. 2. rlhe curve .85 represents the charcteristic of the loop antenna and as indi@ cated this loop may be adjusted so as to favor the reception from the transmitter 10 as regards that from the transmitter 20, the receiver being closer to the transmitter 2O than to the transmitter 30 so that the intensity of the electromagnetic field set up by the latter is lower.

ln the circuit shown in Fig. l the wave observed by the control operator gives an indication of the carriers rather than the programs or side bands. Consequently in order that the best conditions eXist it is necessary that the percentages of vmodulation of the two transmitters be equal. vWhen employing radio transmitters of similar types such a condition can be achieved by adjusting the input levels to the modulating amplifiers so that they are equal. This is accomplished in the system shown in the drawing by adjusting the potentiometers'l) and 29 so that equalindications are given in the indicators 18 and 28. After such an'adjustment hasy been made, the entire control of the amplitude level of the program being transmitted is left in the hands of the operator at the studio. This operator makes the adjustment by con-- trolling the volume 43 and observing the in? dicator 44 in the set up, the operation rof which is described above.

vcontrolling each ofithe. other transmitters by trolledby comparing it with the first con-V an indication from its, respective control transmitter. Similarly one transmitter may be` controlled byr comparing it with a master transmitter and a second transmitter Icontrolled transmitter.

`ArIn vorder to adapt the system to meet a particular situation, it is possible by adjusting the directive characteristics ofthe transmitting antennae to shift thelocation and ch-ange the shape of the interference zone. One method o'f so controlling the directive characteristicsof the antennae is by employingl insulated, tuned towers.

What is `claimed is: Y

1. The method of operating a plurality of radio transmitters at a common carrier frequency which comprises detecting the resultant signals at a point Where the field strengths of the Wavesv from the several transmitters areapproximately equal, transmitting the detected' signals toone transmitter and vadjusting the frequency of the carrier wave'of that transmitter to substantially eliminate the heats from the detected signals.

2; The method of radio broadcasting which comprises operating. a Vplurality ofV radio transmitters on carrier waves of the same kamplitudes of received waves so that they are substantially equal, detecting the received waves to producea signal having a fading,

cycle of frequency proportional to any dierence in frequency between thecarrier wavesV of said transmitters, transmitting the detected signal to one of said transmitters and controlling the frequency of the carrier of said one transmitter to maintain Vthe frequency of the fading cycle at a desired value.'

3. The method of radio broadcasting which comprises operating a plurality of radio transmitters on carrierwaves of the same frequency, modulating the carrier waves of said transmitters by thesame program, receiving the waves transmitted by said1transmitters ata common point, controlling the amplitudes ofY received waves so that they are substantially equal, detecting the received Waves to produce a signal having a fading cycle of frequency proportional to any difference in frequency between the carrier waves of said transmitters, transmitting the detected signal to one of said transmitters, andV controlling the frequency of the carrier of said one transmitter to maintain the period ofthe Vfading cycleatltlie order of to 30" where the field strengths-of the two waves` are approximately equal, modulating thev received waves by an audibley frequency wave.- detecting the resultant combined wave, trans mitting the detected wave/to V.one of said transmitters and controlling the frequency of the carrier of said oiie transmitter in accordance with said detected wave to maintain said transn'iitters in substantial synchro-' nism.

5, The method offbroadcasting a radio programfroin two radio transmitters operating on the same carrier frequency, which method comprises receiving-the two wave-sV lin a directive antenna located intermediate said transmitters, adjusting said anteniia'so that the amplitudes of the two received carrier waves are substantiallyr equal, simultaneously modulating said carrier waves by an audible frequency wave, detecting the re siiltant combined wave, transmitting thedetected wave to oneof said transmitters and:

controlling' the frequency ofthe carrier of said one transmitter 'to hold the"r fading period ofthe detected mined value. Y f

6. The method of radio broadcasting from two radio transmitters which method com# prises'niodulating the carrier wave at one of said transmitters by a program to obtain a modulated wave having a predetermined percentage of modulation, modulating a car-v 'rie'r wave of substantially the same frequency at'the other of said transmittersby the saine program to'obtain a modulated wave having said percentage of modulation, receiving the two modulated waves ina' directive antenna located intermediate said transmitters, arlv VJusting said antenna so that theV amplitudes of the two received carrierv waves are substantially equal, simultaneously modulating said carrier waves by an audiblefrequency f wave, detecting the resultant combined wave.V

transmitting the detected wave to one of said transmitters, andcontrolling the frequency of the carrier of said oner transmitter toV hold the fading period of thefdetected wave at a predetermined value.

7. Incombi'nation, a plurality ofi radio transmitters having Acarrier oscillators operating at .substantially the same frequency," ad]ustable meansvfor giving a sensitive control of the frequency ofthe oscillator one of said transmitters, a source of'signals, means for modulating the carrier wave of each of said transmitters by signals from said source,

a radio receiver for simultaneously receiving waves from both of said Vtransmitters at a f point where vthe field strengthsY produced by wave at a- `predeteriotlliA

said transmitters are approximately equal, said radio receiver including means for detecting the received waves, and means for giving an indication of the detected Waves at said one transmitter.

8. ln combination, a plurality of radio transmitters, each having a crystal-controlled carrier oscillator,V said oscillators operating at substantially the same frequency, adjust able means for giving a sensitive control of the frequency of the oscillatorl of one of said transmitters7 a source of signals. means for modulating the carrier waves from each of said transmitters by signals from said source, a radio receiver for simultaneously receiving waves from both of said transmitters and including means for adjusting the received waves to substantially the same energy level, a source of audible frequency waves, means for modulating the r ceived waves by waves from said source of audible frequency waves and means for detecting the resultant waves, means for transmitting the detected Wave to said one transmitter, and means at said one transmitter for giving an indication of the fading period of said detected wave.

9. rlhe combination according to the next preceding claim, in which transmitters operate to give substantially the same percentages of modulation.

l0. The method of operating three or more radio transmitters in isochronism for transmitting the same signal., which method comprises detecting the resultant signals of two of said transmitters at a point where the strengths of the fields set up by said two tra-nsmitters are approximately equal, transmitting the detected signal to one of said two transmitters, adjusting the frequency of the carrier wave of said one transmitter in accordance with the fading period of said detected signal to maintain said two transmitters in substantial synchronism and similarly maintaining another of the transmitters in substantial synchronism with one of said two transmitters.

In witness whereof, I hereunto subscribe my name this 10th day of Jun-e, 1930.

GLENN D. GLLETT. 

